1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for displaying ultrasonic tomography images of a subject under examination, and more particularly to an ultrasonic diagnostic apparatus adapted to improve image quality which may be degraded by multiple reflections of ultrasonic waves within the body of the subject for the purpose of ensuring high resolution.
2. Description of the Related Art
Ultrasonic diagnostics, in which pulses of ultrasonic waves are transmitted to a living organism and reflected ultrasonic waves from its various tissues are detected to thereby obtain in vivo information of the organism, has made rapid progress recently owing to the development of two techniques called "ultrasonic tomography" and an "ultrasonic Doppler method". An electronic scanning ultrasonic diagnostic apparatus, which nowadays is most widely used, controls an array of ultrasonic transducers at high speed to scan a body region of a subject under examination. Thus, a tomography image of the body region of the subject is displayed in real time.
An ultrasonic diagnostic apparatus which nowadays is in clinical use adopts a pulse reflection method as with conventional radars and sonars. In the pulse reflection method, pulses of ultrasonic waves are directed into a body region of a subject and reflected ultrasonic waves from the body region of the subject are detected to obtain in vivo information of the subject. Basically a tomography image is constructed from the signal strength of each reflected ultrasonic wave and the interval that elapses from the transmission of each ultrasonic pulse to the time when a corresponding reflected ultrasonic wave is received. In this case, ultrasonic imaging is carried out on the assumption that a single reflection occurs for each ultrasonic pulse transmitted.
In fact, however, so-called multiple reflections occur continually within a living organism under examination. Depending on a body region to be imaged, the multiple reflections cannot be ignored. In particular, multiple reflections between a layered tissue near the body surface and an ultrasonic probe or between internal tissues cause image artifacts to be produced on an ultrasonic tomography image, which often constitutes a serious obstacle to diagnosis. This problem will be discussed with reference to FIG. 1.
It is assumed that, as shown in FIG. 1, a layered structure (i.e., tissue boundary surface) of a living body exists at a distance of y from an array of transducer elements constituting an ultrasonic probe and is substantially parallel to the direction in which the transducer elements are arranged. The time when an ultrasonic wave which has undergone two reflections (multiple reflection) at the layered structure is received by the transducer array is 4 y/c (c is the sound velocity in the medium). On the other hand, the time when an ultrasonic wave reflected by a reflector which is at a distance of 2 y from the transducer array is received will also be 4 y/c, which is the same instant of time as when the multiple reflection wave is received.
In the case of a conventional ultrasonic diagnostic apparatus, since the direction of transmission of an ultrasonic pulse and the direction of reception of reflected waves are coincident with each other, it is impossible to separate two reflected waves that are received at the same instant of time. Thus, the multiple reflection wave which has undergone two reflections at the layered structure as shown in FIG. 1 will produce an image artifact which is displayed superimposed upon an ultrasonic image indicating the reflector.
As described above, with the conventional ultrasonic diagnostic apparatus, multiple reflections between tissues forming a layered structure near the body surface and the ultrasonic probe or between internal tissues produce image artifacts in an ultrasonic image, which often makes a serious obstacle to diagnosis.